Courses Getting an Incomplete in a grad course

Hi I'm an undergrad and I decided to take a grad course that i wasn't ready for, and during the first midterm i did horribly, i didn't do well on the homeworks, you get the point. My professor told me the stuff after the first midterm is going to get even more difficult so i should just start attending the undergrad course in mid semester. So I didn't attend, I emailed my professor that I am going to retake the undergrad course next year to get a fresh start.

I know an incomplete is bad, but how bad is it? I have been getting A's for the last 5 semesters in tons of math courses.